Stress Buffer Layer for Ferroelectric Random Access Memory

ABSTRACT

An F-RAM package having a semiconductor die containing F-RAM circuitry, a mold compound, and a stress buffer layer that is at least partially located between the semiconductor die and the mold compound. Also, a method for making an F-RAM package that includes providing a semiconductor die containing F-RAM circuitry, forming a patterned stress buffer layer over the semiconductor die, and forming a mold compound coupled to the stress buffer layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a division of application Ser. No. 12/396,976, filed on Mar. 3,2009, which claims the benefit of U.S. Provisional Application No.61/075,820 filed Jun. 26, 2008, both of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

This invention relates to the packaging of a semiconductor diecontaining Ferroelectric Random Access Memory.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an F-RAM package in accordance withthe present invention.

FIGS. 2A-2D are cross-sectional diagrams of a process for forming anF-RAM package in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is described with reference to the attachedfigures, wherein like reference numerals are used throughout the figuresto designate similar or equivalent elements. The figures are not drawnto scale and they are provided merely to illustrate the invention.Several aspects of the invention are described below with reference toexample applications for illustration. It should be understood thatnumerous specific details, relationships, and methods are set forth toprovide an understanding of the invention. One skilled in the relevantart, however, will readily recognize that the invention can be practicedwithout one or more of the specific details or with other methods. Inother instances, well-known structures or operations are not shown indetail to avoid obscuring the invention. The present invention is notlimited by the illustrated ordering of acts or events, as some acts mayoccur in different orders and/or concurrently with other acts or events.Furthermore, not all illustrated acts or events are required toimplement a methodology in accordance with the present invention.

Referring to the drawings, FIG. 1 is a cross-sectional view of aferroelectric random access (“F-RAM”) package 10 in accordance with thepresent invention. In the example application, the F-RAM package 10includes package substrate 20 and a bonded semiconductor die 30containing F-RAM circuitry. The F-RAM circuitry is located on the activesurface 40 of the semiconductor die 30, as represented by the markings50 for illustration purposes. F-RAM circuitry 50 within thesemiconductor die 30 may be discrete or embedded. Therefore, thesemiconductor die 30 may contain F-RAM circuitry exclusively, or thesemiconductor die 30 may contain F-RAM circuitry plus additionalcircuitry such as a processor.

The F-RAM package 10 also includes contact pads 60 that connect portionsof the F-RAM circuitry 50 to lead tips 70 of the package substrate 20.In the example application, wires 80 (which are typically gold wires)are coupled to the contact pads 60 by balls 90 and also coupled to thelead tips 70 by stitches 100. The wires 80 electrically connect theF-RAM circuitry 50 of the semiconductor die 30 to the package substrate20.

The F-RAM package 10 further includes mold compound 110 (such as epoxyor resin) that protects and encapsulates the objects located on thesurface of the package substrate 20. The mold compound 110 iselectrically insulating, moisture-impenetrable, and mechanically strong.Generally, the mold compound 110 is mechanically strong because itincorporates silicon or silicon dioxide particles. In addition to addingmechanical strength, the particles adjust the thermal expansioncoefficient (“CTE”) of the mold compound so that it better matches theproperties of the integrated circuit.

An unsolved problem with F-RAM circuitry that is experienced in themarketplace is an undesirable level of clustered weak bits. The presenceof cluster bits reduces the data signal margin (e.g. the ability of thesense amp to read a bit due to reduced switch polarization) and resultsin weakened ferroelectric memory bits that have a shortened time tofailure.

While working to determine the cause of the cluster bit problem, theinventors theorized that the silicon or silicon dioxide particles thatare contained within the mold compound 110 created regions of localizedcompressive stress on the surface of the semiconductor die 30. Thattheory resulted from their investigation showing that the cluster bitsdisappeared when the mold compound 110 was removed from locations abovethe F-RAM circuitry 50 of the semiconductor die 30. Furtherinvestigations by the inventors revealed that the footprint of theclustered bits was similar to the outline of the particles within themold compound—taking into consideration the fact that stress isgenerally transmitted outwards at a 45° angle through F-RAM circuitry50.

The data from those investigations also showed that the diameter of thecluster bit regions (i.e. the cluster bit footprint) matched thethickness of all material layers located above the ferroelectriccapacitor level of the F-RAM circuitry 50. Specifically, theferroelectric layer of the F-RAM circuitry 50 is generally located about3 μm below the top surface of the semiconductor die 30 and the inventorsobserved that the typical size of the cluster error bits was about 6 μmwide with a generally circular shape. The regions of localizedcompressive stress were eventually attributed to the switchpolarizations problems observed with the F-RAM circuitry 50.

In summary, it was theorized by the inventors that the cluster bits arecaused by the silicon or silicon dioxide particles in the mold compoundthat are in physical contact with the surface of the F-RAM circuitry 50.Experiments showed the inventors that the differential stress on themold compound exerted a point stress originating from the silicon orsilicon dioxide particles onto the surface of the semiconductor die 30.The point stress from the silicon or silicon dioxide particles created alocal compressive stress that spread outwards below the surface of thesemiconductor die 30.

The inventors' solution to the issue of mold compound compressive stressis the use of a dielectric stress buffer layer 120 that is locatedbetween the F-RAM circuitry 50 of the semiconductor die 30 and the moldcompound 110, as shown in FIG. 1. The stress buffer layer 120 has athickness greater than 3 μm and is preferably between 3-20 μm thick.

The inventors' solution to the issue of mold compound compressive stressis the added use of a dielectric stress buffer layer 120 that has anelastic modulus that is less than the elastic modulus of materials thatare currently located above the layer of ferroelectric material withinthe semiconductor die 30 (such as SiO₂, SiN, and Cu). Therefore, thestress buffer layer 120 of the present invention has an elastic modulusbelow 10.5×10⁶ psi. In the example application shown in FIG. 1, thestress buffer layer 120 has an elastic modulus below 2×10⁶ psi.

It is to be noted that windows that are etched through the stress bufferlayer 120 expose selected locations within the semiconductor die 30, asshown in FIG. 1 and described infra. In the example application, thestress buffer layer 120 is removed above the contact pads 60 tofacilitate the formation of wires 80 between the contact pads 60 and thelead tips 70.

In the first embodiment of the present invention, the stress bufferlayer 120 is a photopatternable silicone. Examples of commerciallyavailable photopatternable silicone 120 are photodefinable spin-onsilicones (such as Dow Corning3 s WL-5000 Series® silicones) and waferpermanent resist (such as DuPont's PerMx Series® microlithographicpolymer films). Photopatternable silicone is applied to thesemiconductor die 30 using low temperature processes (e.g. under 250°C.). Therefore, the low thermal budget requirements for F-RAMfabrication are not violated (preventing the degradation of theferroelectric polarization within the F-RAM circuitry).

However, it is within the scope of the invention to use other materialsfor the stress buffer layer 120. For example, in an alternativeembodiment of the present invention, the stress buffer layer 120 isphotosensitive polyimide (“PI”), which is commercially available frommany manufacturers (and it has a higher cure temperature of <400° C.) .In another alternative embodiment of the present invention, the stressbuffer layer 120 is photosensitive benzocyclobutenes (“BOB”) (availablefrom Dow Chemical® under the trade name Cyclotene). In yet anotheralternative embodiment of the present invention, the stress buffer layer120 is poly-p-phenylenebenzobisoxazole (“PBO”), which is commerciallyavailable from Sumitomo Bakelite Company. Experiments conducted by theinventors showed that the presence of PBO between the F-RAM circuitry 50and the mold compound reduced the occurrence of cluster bits within theF-RAM circuitry 50. In one trial, a yield improvement of over 25% wasattained with the use of PBO as a stress buffer layer. The inventorsbelieve that the stress buffer layer 120 lowers the point stresses ofthe particles within the mold compound to a level where the particleshave minimal negative impact on the ferroelectric properties of theferroelectric layer within the F-RAM circuitry 50.

Referring again to the drawings, FIGS. 2A-2D are cross-sectional viewsof a partially fabricated semiconductor wafer 150 illustrating a processfor forming the stress buffer layer 120 of FIG. 1 in accordance with thepresent invention. The uncut semiconductor wafer 150 contains F-RAM diesegments 35 and scribe locations 160 (where the wafers are later sawedinto individual dies 30). The following example application is exemplarybut not restrictive of alternative ways of implementing the principlesof the invention. Moreover, features and procedures whoseimplementations are well known to those skilled in the art are omittedfor brevity. For example, the implementation of common fabrication stepslies within the ability of those skilled in the art and accordingly anydetailed discussion thereof may be omitted.

FIG. 2A is a cross-sectional view of an uncut semiconductor wafer 150 atan intermediate step in the integrated circuit fabrication process. Asnoted supra, each semiconductor die segment 35 of the semiconductorwafer 150 includes discrete or embedded F-RAM circuitry 50 that islocated within its active silicon surface. At this stage in thefabrication process, the uncut semiconductor wafer 150 also includescontact pads 60 and scribe lines 160.

Next, a patterned stress buffer layer 120 is formed over the uncutsemiconductor wafer 150. The first step in forming the patterned stressbuffer layer 120 is the spin coating of a conformal film of the stressbuffer material 130 across the top surface of the semiconductor wafer150, as shown in FIG. 2B. In accordance with the present invention, thestress buffer layer has a thickness greater than 3 pm and an elasticmodulus less than 2×10⁶ psi. The precursor of the stress buffer material130 in this example embodiment is photoimageable. Any standard machine(such as the ACT 8 sold by TEL®) may be used to form the stress bufferfilm 130.

After a standard development process, a patterned stress buffer material120 is formed over the uncut semiconductor wafer 150, as shown in FIG.2C. In this example application, the patterned stress buffer layer 120exposes the contact pads 60, input/output terminals (not shown), andscribe lines 160. Any developer recommended by the supplier of thestress buffer material may be used to pattern the photoimageableconformal stress buffer film 130.

It is to be noted that in fabrication processes where the stress buffermaterial 130 is not photoimageable, a patterned photoresist 140 may beused as a mask for developing the stress buffer film 130, as shown inFIG. 2D. If used, the patterned photoresist 140 will designate whichportions of the non-photoimageable stress buffer layer 130 will beremoved through a patterning process that uses ultra violet light. Thepatterned photoresist 140 is removed (before the next process step) witha standard ash and clean process, resulting in the structure shown inFIG. 2C.

Once the patterned stress buffer layer 120 shown in FIG. 2C is formed,it is cured. In one embodiment, the cure is done in a standard furnace(such as the Alpha sold by TEL®) at a temperature between 150-350° C. Inanother embodiment, the patterned stress buffer layer 120 is cured attemperatures between 150-250° C. to guard against an unacceptablereduction in the data signal margin of the circuitry 50. In thisalternative embodiment, the cure may be performed in a rapid thermalprocessing (“RTP”) tool (available from companies such as LambdaTechnologies) having the capability of producing variable frequencymicrowaves. The use of cure temperatures below 250° C. also minimizesthe effect of CTE mismatches between the various materials comprisingthe semiconductor wafer 150.

The semiconductor wafer 150 is now ready for standard assembly, test,and packaging processes. In general, those processes may includedepositing a standard mold compound 110 over the semiconductor wafer 150and sawing the uncut semiconductor wafer 150 along the scribe lines 160to create individual semiconductor dies 30. Each semiconductor die 30 iseventually attached to a package substrate 20 where wires 80 are addedto form electrical contacts between the semiconductor die 30 and thepackage lead tips 70 (as shown in FIG. 1).

Various additional modifications to the invention as described above arewithin the scope of the claimed invention. As an example, instead offorming the conformal film of the stress buffer material 130 across thesurface of a semiconductor wafer 150, the stress buffer layer 120 may beapplied after the semiconductor wafer 150 has been sawed (and individualdies 30 created). In that alternative fabrication process, the stressbuffer layer 120 would be formed by a process that dispenses a drop ofthe liquid stress buffer layer precursor onto the semiconductor 30 afterit has been mounted onto the package substrate 20 (e.g. before the moldcompound 110 is applied). Alternatively, smaller or more rounded siliconor silicon dioxide particles may be used within the mold compound 110 toreduce the cluster bit problems caused by the mold compound 110. Forexample, Hitachi CEL 9510HF10 mold compound is highly filled (i.e.approx. 88%) with exclusively spherical particles.

In addition, the package substrate 20 of FIG. 1 may be the chip mountpad of a prefabricated lead frame. Alternatively, the package substrate20 may be a ball grid array (“BGA”). Moreover, semiconductor die 30 maybe flip-chip bonded (using bumps and underfill material) to the packagesubstrate 20. Furthermore, more than one semiconductor die 30 may beelectrically coupled to the package substrate 20.

It is also within the scope of the invention to have additional windowsthrough the stress buffer layer 120 to the F-RAM circuitry 50 in orderto provide wire bonding access to additional circuitry - such as powersupplies and input/output terminals. In addition, the stress bufferlayer 120 may contain more than one material. For example the stressbuffer layer may contain a PBO film plus a photopatternable siliconefilm. Alternatively, a sacrificial inorganic protective film may bedeposited over the surface of the uncut semiconductor wafer 150 beforeapplying the stress buffer film 130. In this alternative embodiment, thethickness of the sacrificial inorganic protective film may be less than1000 Å thick and it may contain silicon nitride, silicon dioxide, orphosphorous silicon dioxide. (This sacrificial inorganic protective filmmay be removed with a dry etch process after the patterned stress bufferlayer is cured.) Moreover, the methods described supra may include oneor more cleaning processes for optimum fabrication yield.

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, and not limitation. Numerous changes to the disclosedembodiments can be made in accordance with the disclosure herein withoutdeparting from the spirit or scope of the invention. Thus, the breadthand scope of the present invention should not be limited by any of theabove described embodiments. Rather, the scope of the invention shouldbe defined in accordance with the following claims and theirequivalents.

1. A method for making an F-RAM package, comprising: providing asemiconductor wafer that includes semiconductor die segments containingF-RAM circuitry; forming a patterned stress buffer layer over saidsemiconductor wafer; and forming a mold compound coupled to said stressbuffer layer.
 2. The method of claim 1, wherein said step of formingsaid patterned stress buffer layer is performed at temperatures below250° C.
 3. The method of claim 1, wherein said stress buffer layer iscomprised of photopatternable silicone.
 4. The method of claim 1,wherein said stress buffer layer is comprised ofpoly-p-phenylenebenzobisoxazole.
 5. The method of claim 1, wherein saidstress buffer layer is comprised of photosensitive polyimides.
 6. Themethod of claim 1, wherein said stress buffer layer is comprised ofphotosensitive benzocyclobutenes.
 7. The method of claim 1, wherein saidstress buffer layer has a thickness greater than 3 μm and an elasticmodulus less than 2×10⁶ psi.
 8. The method of claim 1, wherein saidsemiconductor die is a discrete F-RAM device.
 9. The method of claim 1,wherein said semiconductor die also contains a processor.
 10. The methodof claim 1, wherein said step of forming said patterned stress bufferlayer comprises: forming a stress buffer film over said semiconductorwafer; applying a patterned photoresist over said stress buffer film;using said patterned photoresist as a mask to form said patterned stressbuffer layer; and removing said patterned photoresist.